It's a bit of a long shot, but (if you can see anything at all) you might try uninstalling your video drivers and then installing the latest version from your card's or motherboard's manufacturer. If your monitor is completely dark but your system sounds as though it's loading, your monitor could be shot (or it could be set to display the wrong connection type), your video card could be dead, your power supply could be misbehaving (if it supplies juice to the card, disconnect and reconnect it to see if that fixes things), or your SLI connector may be awry.
My System Randomly Restarts Without a Crash or a Blue Screen
Though not always helpful, the fabled Blue Screen of Death may offer a clue about the underlying problem. Completely random restarts (or spontaneous shutdowns) are a bit more vexing.
One possible explanation is that components within your system are overheating. You'd typically see a graphical abnormality if your graphics board were at fault, but not if your CPU or a hard drive were getting too hot. There are several ways to help keep your CPU cool: Clean your CPU cooler; add more fans to your case; clean the thermal paste off your CPU and reapply a fresh drop. If your hard drive is to blame--and it isn't failing outright--get additional air on it or clean the fan (or fan grills) already directly responsible for cooling it.
Your power supply may be on the verge of giving out, but there's no good way to test its overall stability without replacing it and seeing whether the problem then goes away. Another possibility is that your power supply simply can't handle your components at maximum load--if you're trying to run a severely underpowered PSU on a tri-SLI setup, for example, trying to play a resource-hungry game might cause your power supply to wave the white flag. Of course, it might just be that your power supply's cables are loose.
Yet another potential sticking point is memory. Try removing and reinserting memory modules to see whether the problems disappear. A system with dual-channel memory may have the minimum two memory modules. If so, move those modules to the alternative channel slots.
All in all, the likeliest source of trouble is something related to your PC's power supply--but again, generic issues like "computer resets with no warning" resist simple, one-approach-fixes-all solutions.
My Keyboard Isn't Working--Nothing I Type Comes Up on the Screen
If you're trying to use a USB keyboard before Windows boots up, make sure that you've enabled support for legacy USB devices within your BIOS settings.